Cotton is an important and valuable field crop which is used to manufacture textile products, oil, animal feed, cordage and other non-woven products. Cotton production today is based mainly on cultivation of varieties of the species Gossypium hirsutum, known as Upland cotton. These cotton varieties are generally preferred for their high lint yield potential, early maturity, and adaptation to adverse climatic and growing conditions. On the other hand, the quality of Upland cotton lint is considered low to medium.
Varieties of another species, G. barbadense, known as Pima cotton, constitute only 5-8% of the world cultivated cotton area. Pima varieties typically produce superior lint having long, strong and fine fibre. On the other hand, these varieties usually have low yield potential, require a long growing season, and can only be cultivated in warm regions. Cotton lint quality is measured by a number of measures including fibre length, strength and micronaire. Accordingly, the lint quality is considered higher when the fibre is longer, stronger and finer when the fibre is fully matured in open bolls.
One of the main constraints on cotton production worldwide is the damage caused by insect pests such as the cotton bollworm (Helicoverpa sp.). In the last 20 years, cotton has been genetically engineered by the insertion of transgenes encoding insecticidal proteins from Bacillus thuringiensis (Bt), thereby providing in planta production of the Bt proteins in leaves and bolls and a degree of protection against the insect pests. This has resulted in a substantial decrease in the use of chemical insecticides by spraying, with environmental benefits. However, the emergence of insect pests which are tolerant to the Bt proteins remains a concern and integrated pest management strategies including the use of, for example, non-transgenic refugia are important. More recently, the adoption of cotton cultivars expressing two Bt proteins with different modes of action has been an important development. For example, the Bollgard II varieties incorporate transgenes for expression of Cry1Ac and Cry2Ab proteins and have been grown in various countries including Australia for 10 years. However, the potential emergence of pest populations having resistance to the Bt proteins remains a concern (Downes et al, J. Invertebrate Pathol. 110:281-286, 2012).
An alternative, non-Bt insecticidal protein, Vip3A, has been proposed to be combined with the dual Bt proteins. Cotton plants expressing Vip3A have been produced. However, expression levels of the Vip3A protein declined as the season progressed, leading to concerns about its effectiveness (Llewellyn et al, Agric. Forest Entomol. 9:93-101, 2007).
Transgenic cotton cultivars engineered for insect pest tolerance also need to be agronomically suitable and capable of producing lint at high yield and quality, to be commercially acceptable. This is a great challenge to cotton plant breeders when introducing the transgenes into locally adapted varieties. Due to the environment, the complexity of the structure of genes and location of a gene in the genome, among other factors, it is difficult to predict the phenotypic expression of a particular genotype in different genetic backgrounds. In addition, plant breeding applies to the phenotype and not on the level of the genotype. Therefore, a newly bred variety is considered to be an unexpected result of the breeding process. In particular, each variety will typically contain a unique combination of known and novel characteristics, based not just on the introduced transgenes but also to the totality of the genetic background.
There remains a need for well adapted cotton varieties with in planta insect tolerance and which produce lint at high yield and quality.